Analytical Industries GPR-1800 IS ATEX Trace PPM Oxygen Transmitter - Manual (Page 25)

Advanced Instruments, Inc
25
exposure to high O2 concentrations or air is significantly different if a sensor is being installed than if the
sensor had been in-service at low oxygen levels for more than 1 week.
Sensor Calibration at
Install In-service
Calibration
PPM
Fuel
Cell
Air to .1% < 1
min
Air to 100 PPM <
5 min
Air to < 10PPM <
60 min
Similar
Less than 45 min
The above times assume the introduction of a zero gas (low level of oxygen in nitrogen) after span
calibration.
For optimum accuracy, the O2 concentration of a span gas should be approximate 50-90% of the full scale
range of analysis or one range above the analysis range, e.g. 20.9% O2 on the 0-25% range. Conversely, if
the recommended span gas is not available and air calibration is not an option, a span gas of the same full
scale range and near the anticipated analysis level (approximately 30-50% of full scale) is acceptable with
the understanding that the accuracy will suffer slightly.
Use of span gas near 30% of the full scale range of measurements, at the higher end of the range has the
effect of “expanding the error” as illustrated by Graph A in Example 1 in the Accuracy section above.
Prematurely initiating the SPAN CALIBRATION function (before the analyzer reading has stabilized) can
result in erroneous readings as follows:
When purging an analyzer to lower ranges and calibrating with a span gas: If the oxygen reading reaches
less than 2% of the intended calibration range, enter the value of the span gas. If the oxygen reading is
greater than 2% of the calibration range, add the O2 reading to the value of the span gas (the impact of the
offset on accuracy is minor but the addition allows the oxygen sensor to continue to purge down and avoid
negative readings after calibration.
When installing a new oxygen sensor and calibrating with air, allow 2-3 minutes for the sensor to equilibrate
in ambient air from storage packaging. Failure to do so can introduce error in calibration.
Zero Calibration
Typical offset from a PPM analyzer is less than 0.5 PPM. Therefore, for most applications, a Zero calibration
is not required. However, ZERO calibration feature has been provided to allow the user to precisely measure
oxygen concentration at the very low levels (less than 0.5 PPM). As described below, accomplishing either
objective places a degree of responsibility on the user.
Determining the true offset requires the user to wait (see Online Recovery Time section) until the analyzer
reading is no longer trending downward (best evidenced by a constant horizontal trend on an external
recording device.
The zero offset adjustments is limited to 50% of the most sensitive range of the analyzer. At factory,
analyzer is QC tested to confirm that the maximum offset is less than 50% of the most sensitive range
available. Should you observe a zero offset more than 50% of the lowest range, check sample system for
any possible leaks, integrity of the zero gas and assure that the analyzer has been given enough time to
stabilize on zero gas before initiating the ZERO CALIBRATION.
Caution: If adequate time is not allowed for the analyzer to establish the true baseline and a ZERO
calibration is performed, the analyzer will in all probability display a negative reading in the sample mode
after a certain period of time. If a negative reading is seen, perform ZERO calibration again.